That, for the time being, is for the future. At present, the evidence we have is that the planetary systems found so far have their planets in the wrong place and in the wrong orbits. If you are a theorist and are putting together a model or explanation for some phenomenon, then you feel that you are doing pretty well if you are able to account for more than around 90-95% percent of the observational "facts." Actually, some of us feel we are doing well if we can manage half of them. With this in mind, there have been two main approaches to developing theories of planetary formation. The first, and the most popular, is to follow the advice in Hilaire Belloc's poem, which is "Always keep a-hold of nurse, for fear of finding something worse." The second, is to take the Monty Python refrain "And now for something completely different." Let's look at the second first.
4.1. The Monty Python approach The main alternative strategy is to try to form planets directly, using gravity alone. It seems to work for stars, so why not for planets? Using this approach one would expect to find only gas giants—thus no terrestrial planets and not much astrobiology. But, except for the Solar System, this would not contradict what we currently know. For example, there is little evidence as yet that extra-solar planets have cores.
The current status of this approach was well summarized in the talk by Richard Durisen. The point here is that since stars form with angular momentum, it seems likely that most of the mass of a star is, at some stage, processed through the disk. Thus, in the early stages at least, the disks should be self-gravitating. Most of the computational simulations of this process start with a sufficiently massive disk around a central point mass that it is self-gravitating and then let go. This is, of course, not realistic, but numerically probably the best one can do. Some authors find that planet-sized objects condense out, and some do not. But the general consensus at present appears to be that the generation of long-lived planetary mass objects has yet to be demonstrated. The major problem appears to be that, in order to get self-gravity to produce individual objects, rather than just produce spiral arms which accelerate the accretion process, it is necessary for the gas to be able to cool rapidly. Thus, good, accurate treatment of radiative processes will be required. There are also numerical issues. Modeling the evolution of compact structures in an accretion disk calls out for the use of a Lagrangian numerical method such as SPH—grid methods are not well suited to following compact structures moving supersonically across the grid. However, radiative transfer is more easily accurately done in a grid environment.
The basic conclusion seems to be that so far the viability of this approach is "not proven." The signs at present do not look encouraging, but there are still a lot of possibilities to explore.
4.2. Keeping a-hold of nurse As we have heard at this meeting, most work on the formation of planetary systems involves keeping "a-hold of nurse." That is, the favored strategy is to take the original set of ideas for the formation of the Solar System and set about modifying them to fit the extra-solar data. A glance through the various review talks presented here shows that there has been a lot of progress. Understandably, most speakers have emphasized what works. I'll focus here a bit more on what doesn't work, and what needs to be done next.
There are basically two sets of problems here—problems with the Solar System story, and problems with the modifications required.
The general narrative of planet formation in the Solar System contains a number of miracle moments. The general aim is to start with a gaseous disk of material, allow the heavier elements to condense out as dust, to allow the dust to settle and coagulate into ever larger entities going from pebbles to rocks to planetesimals, and thence to planetary cores. The details of many of these processes still remain obscure. Having formed planetary cores, it is then necessary to persuade the gas to accrete onto it (to form gas giants) fast enough, before the disk has dispersed. This process was discussed in the talks by Calvet, Lissauer and Hubickyj. Another problem here, mentioned briefly by Armitage, is the one of Type I migration. This is the regime when a small planet, or protoplanet, has formed in the disk and is not massive enough to open up a gap in the surface density. Although the estimates of the migration timescales (both analytic and numerical) for this process have been steadily increasing, the current values of the migration timescales are still too short, by about an order of magnitude, to fit comfortably with observations.
The main modifications which need to be made to this standard picture are two-fold. First, we require radial migration in order to form the gas giants in the standard place and then move them inwards to where they are found. Second, we need to generate the observed eccentricities.
In general, as shown in the talk by Phil Armitage, and commented upon by Marcy, the migration story seem to work fairly well (as long as one can somehow suppress Type I migration) and the radial distribution of the currently observed systems can be reasonably accounted for. The more serious problem seems to be the one of generating the required distribution of eccentricities. The basic problem here is that accretion of gas to form gas giants and radial migration both require gas in the disk, and a planetary orbit in a gaseous disk that becomes and remains close to circular. And yet, except for the close-in planets, the mean eccentricity is in the range 0.2-0.3, with no evidence for dependence of eccentricity on orbital size. The most favored explanation, mentioned by Marcy, Lin, and others, is to somehow pump the eccentricities once the gas has gone. Probably the only hope for this is to invoke some kind of non-linear dynamics or long-term chaos which can come take effect on a timescale much longer than that required for planet formation and disk dispersal.
Can we decide between the Belloc and Python approaches to planetary formation? Or does the truth lie somewhere in between? Until the discovery of Earth-like planets elsewhere, or the finding of high metal abundance in some observed gas giant, there is, as yet, no direct evidence for the occurrence of core accretion. But there are two sets of observations which could be the key to discriminating between the two.
First there is the finding, reported here by Jeff Valenti, that the presence of a planetary system around a star is strongly dependent on the metal abundance of that star, with the probability varying approximately as the square power of the metallicity. Since the core accretion model depends entirely on the presence of metals, this is really sine qua non for that model. But, as we have seen, the gravitational instability model (if it works at all) depends crucially on cooling processes in the disk, which are also likely to depend strongly on heavy element abundance. Thus, both models can probably be brought into line with this result, but as yet the details have yet to be worked out.
Second, there are the dust/debris disks reported on by Mike Meyer and Lynne Hillenbrand. The very fact that there does seem to be dust and other solid debris in the form of a disk presumably left over from the star formation process indicates that such dust and debris can indeed form, even if the theories as to how it actually does so are not yet complete. The statistics have yet to come in on frequencies and lifetimes, but the very existence of these immediately suggests that at least some parts of the core-accretion scenario seem to ring true. But to be fair, even if one did form many or most of the observed planetary systems through self-gravity, there is no reason to suppose that what is left over might not have formed a debris disk. Indeed, it is the imaging of these disks which might be able to give us clues as to how far from the central star planets can actually form, or end up. Not discussed at the meeting was the body of work (e.g., Wyatt et al. 1999; Quillen 2005; Wyatt 2005) on the observed structure in debris disks which can be interpreted as being caused by Jupiter-mass objects at distances of many tens to hundreds of AU from the central star. If gas giant planets are indeed present at these distances, then that would substantially extend the parameter space of observed planetary systems well beyond those plotted in Figure 1. Ironically, this would also give a headache to the standard core-accretion scenario, because the timescales for forming gas giants at such large radii greatly exceed the dispersal timescales for gaseous disks. Indeed, in answer to a question after his talk, Doug Lin went so far as to claim that if a planet was found that far from the central stars, then his models could not explain it. I am sure that is not true. At these distances, however, as the presentation by Schneider of a —5 Jupiter-mass object some tens of AU from a —25 Jupiter-mass brown dwarf shows, we start having to address the question of what is a planet, and what is not.
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